Structural safety analysis of old timber structures is based on gathering step-bystep
information about the characteristics of the wooden structural elements
leading to allocation of residual strength values. Although this task is often
supported on a probabilistic basis (strength Probability Distribution Function of a
certain visual strength grade) turns out quite deterministic when strength values
allocation is carried out in an global (structure) or local (member) manner by
assignment of the same strength value to different structural timber members
(showing different defects and/or the same defect but with different levels of
magnitude) based on the 5th percentile corresponding to a certain visual strength
grade population.
This simple approach leads in most cases to over conservative values
resulting in recommendations for the replacement of old timber structures after
carrying out safety structural analysis following current design standards
(Eurocode 5). This outcome was observed while performing safety analysis on
different historical timber constructions in Portugal (roofs and flooring systems)
which stayed in service for more than 100 years and showed incipient levels of
deterioration.
Having in mind the need to maintain historical timber structures a new
approach for allocating strength values to individual timber members is
presented and discussed. This approach is based on the experience and data
obtained during the development of visual grading standards and the use of new
technologies available nowadays (for instance probing and ultrasounds). The
possibility of combining information from new and old technology will be
evaluated by testing pine timber beams and by application to a case study.
Keywords: timber, structures, survey, resistance, ultrasounds.